Fontainebleau was targeted only four years ago. Some 20 priceless artefacts were taken and never recovered.

Police say the trail in that inquiry has gone cold.

How the suspects were caught

Spanish police were first alerted to a potential plot to target a French national museum specialising in Asian art.

They passed information to Europol, along with names and photos of some of the suspects.

The case was taken on by France’s central office for combating the trafficking of cultural goods (OCBC). Six suspects were located staying in a hotel in Nemours, 16km (10 miles) south of the palace.

A French surveillance team tracked the suspects for several days. Trips were made to the palace and a series of objects were bought, including gloves, hoods, pickaxes and crowbars. Two cars were stolen, according to police.

Then in the early hours of 28 December, police moved in on the group in the hotel car park.

“We found photos that proved for us they were going to steal works of art of Asian origin that were on exhibition in the museum,” Col Didier Berger of the OCBC told French media.

What we know of the alleged plot

The six suspects deny the allegations against them and insist they were staying in the area as tourists.

He added: “Right now I can’t say whether it was triads or mafia, but it’s a very serious lead for us.”

It is too early to say if the alleged crime was commissioned by a group specialised in art theft.

“Was it ordered and ordered by an Asian country? We can’t say that now,” commander Boyer said.

Image captionFontainebleau palace was home to generations of ruling French dynasties

Although the theory of a Chinese link is being actively investigated, another Asian country such as South Korea could equally be involved. Police are not even sure the sixth man in custody is Chinese as he had false papers from a foreign country.

Of the five Spanish suspects, some already have criminal records. Spanish police sources have already told national media that a seasoned criminal called Juan María Gordillo Plaza, also known as El Niño Juan, is among them.

One police officer was quoted by Journal de Dimanche as saying the order must have come from a Chinese triad. “Chinese mafia have a big presence in Spain and have big economic set-ups and enormous financial means.”

Image captionA replica of a crown of the King of Siam, given to the French emperor in 1861 and stolen in 2015

What is at Fontainebleau?

The Chinese Museum there was built to house the Empress Eugenie’s Far-Eastern treasures.

Some were, in the museum’s words, revolutionary plunder from the imperial depository and other artefacts were seized when the Summer Palace in Beijing was sacked by French and British soldiers in 1860.

The artefacts stolen in 2015 included a replica of a crown of the King of Siam, now Thailand, given to Emperor Napoleon III in 1861.

The stolen pieces are now thought to have been moved abroad. The French team says some 90% of stolen art leaves France – and it leaves the country fast.